1. Field of the Invention
The invention relates to optical scanners and, more particularly, to folded optical scanners of the unidirectional flying-spot type for scanning a document line by line.
2. Prior Art
Flying spot optical scanners illuminate successive segments of a line on a document with a beam of light of small areas measured by the resulting reflectance of the document to thereby provide a signal representative of information contained in the document. Unidirectional scanners sweep the spot across the document in a single direction.
One common system for illuminating a document uses a rotating scan mirror which receives a beam of light from a stationary source and, through its own motion, converts the stationary beam to a moving beam which traverses the document being scanned. One or more lens elements are generally interposed between the scan mirror and the document being scanned in order to focus the scanning beam onto a small area. In such systems, it is deisrable to utilize a long focal length for the main lens in order to limit the angular convergence imposed on the lens (i.e. the angle through which the scanning beam must pass in illuminating the line) to thereby limit optical aberrations caused by large off-axis angles of illumination. However, long focal lengths increase the dimensions of the scanning system and this is generally undesirable.
Folded optical systems (i.e., those in which the light path is bent back on itself) are sometimes used to provide an effectively long focal length while limiting the space the system occupies. However, in such systems the scan mirror and the document cannot be so arranged that the illuminating beam remains in a plane containing the desired straight-line scan line and thus the scan line is not straight, as desired, but is instead bowed. The longer the scan line, of course, the greater the bow (departure of the actual scan line from the desired scan line).
Astigmatic effects associated with field curvature also "spread" the illuminating beam and thereby limit the resolution that can be obtained from optical systems. Further, unwanted reflections from optical elements such as lenses and mirrors frequently cause spurious inputs to the photoresponsive element.
This is especially a problem in retro-reflective optical systems, that is, those in which the reflected beam returns along the same path as the illuminating beam, since the return beam is generally of greatly diminished intensity with respect to the illuminating beam. Under these conditions, undesired reflections ("ghosts") of the illuminating beam from the surfaces of lenses or other optical elements may degrade, or even totally mask the return information-bearing beam.